Fission type nuclear reactors may be configured to use a neutron moderator to slow down, or moderate, neutrons produced by nuclear fission in order to increase the cross-section of the fuel source. The increased cross-section may in turn increase the number of neutrons that are available to cause fission events, rather than being captured by the fuel source, and thus propagate an ensuing chain reaction of fission events.
A thermal neutron is a free neutron which may have a kinetic energy of about 0.025 eV and/or a speed of 2.2 km/s after having a number of collisions with nuclei in a moderator, for example, at a temperature of approximately 17 degrees Celsius. Thermal neutrons typically have much larger interaction cross-sections than fast neutrons, and therefore may be more readily absorbed.
A combination of different types of neutron moderators, moderator temperatures, fuel cross-sections, and/or fuel temperatures may affect the rates of fission that are achievable during reactor startup and/or during operation of the reactor. For example, an increase in fuel temperature may raise the rate of epi-thermal neutron absorption of the fuel and provide a negative feedback that may be used to control the power level of the reactor. Additionally, a change in moderator temperature may also be used to provide negative feedback.
A device configured to emit neutrons, such as a neutron source, may be designed with a number of different parameters in mind. For example, the neutron source design parameters may include an amount of energy of the emitted neutrons, an emission rate of the neutrons, and/or other parameters depending on the particular application of the neutron source and/or of the reactor.
Spontaneous fission events produced by the fuel may be too weak for certain types of reactor monitoring instrumentation to detect. Starting a reactor without knowing the level of fission events and/or the level of neutron flux at or near the reactor core may be referred to as a “blind” start, which may not be permissible under various regulatory and/or operational requirements.
Neutron capture resulting from the thermal neutron flux in an operating reactor may change the composition of the isotopes, and reduce the useful life of the neutron source. Accordingly, the neutron source may be changed or replaced at regular intervals to ensure that there remain a sufficient number of neutrons being emitted during startup and/or during operation. While some types of neutron sources which are considered inert may be less expensive than neutron sources which are active, the initial absence of a sufficient neutron flux from the inert neutron source may result in a blind start. Additionally, some types of neutron detectors located at or near the reactor core may be configured to detect high levels of neutrons during reactor operation, and may not be sufficiently sensitive to detect relatively low levels of neutrons and/or to accurately measure reactivity, e.g., at reactor shutdown or at reactor shutdown.
A neutron source which does not have and/or which loses the capability to generate a sufficient number of neutrons in one or more modes of reactor operation may result in the reactor monitoring instrumentation being unable to detect or confirm the presence of the neutron source and/or to verity the associated neutron activity. Additionally, in some examples the inability to detect the level of neutron activity could also affect the ability to monitor an unexpected increase in reactivity during core shutdown, inspection, maintenance, and/or refueling.
The present application addresses these and other problems.